1. Field of the Invention
The present invention relates generally to spacecraft propulsion systems and, more particularly, to ion thrusters.
2. Description of the Related Art
On-board propulsion systems facilitate a number of spacecraft maneuvers. In satellites, for example, these maneuvers include orbit raising (e.g., from a low Earth orbit to a geostationary orbit), stationkeeping (e.g., correcting orbit inclination, drift and eccentricity) and attitude control (e.g., correcting attitude errors about a satellite's roll, pitch and yaw axes).
The force exerted on a spacecraft by a propulsion system's thruster is expressed in equation (1) ##EQU1## as the product of the thruster's mass flow rate and exhaust velocity. Equation (1) also shows that mass flow rate can be replaced by the ratio of weight flow rate to the acceleration of gravity and that the ratio of exhaust velocity to the acceleration of gravity can be represented by specific impulse I.sub.sp which is a thruster figure of merit. Equation (1) can be rewritten as equation (2) ##EQU2## to show that specific impulse is the ratio of thrust to weight flow rate.
A velocity increase .DELTA.V of a spacecraft is gained with a loss in mass of the thruster's stored fuel. Accordingly, a differential occurs between the spacecraft's initial mass M.sub.i (prior to the maneuver) and the spacecraft's final mass M.sub.f (after the maneuver). This mass differential is a function of the thruster's specific impulse I.sub.sp as expressed by the "rocket equation" of ##EQU3## in which .DELTA.V has units of meters/second, I.sub.sp has units of seconds and acceleration of gravity g has units of meter/second.sup.2. Equation (3) states that fuel use causes a spacecraft's final mass M.sub.f to exponentially decrease with increased .DELTA.V and that this decrease can be exponentially offset by an increase in specific impulse I.sub.sp.
Therefore, specific impulse is an important measure of a thruster's fuel efficiency. Typical specific impulses are 230 seconds for monopropellant (e.g., hydrazine) thrusters, 290 seconds for solid propellant thrusters, 445 seconds for bipropellant (e.g., liquid hydrogen and liquid oxygen) thrusters and 500 seconds for electric arc jet thrusters. In contrast, ion thrusters have been developed with specific impulses in excess of 2500 seconds.
The high specific impulse of ion thrusters can facilitate a reduction of initial satellite mass, an increased payload and a longer on-orbit lifetime. Reduction of initial mass lowers the spacecraft's initial launch cost and increased payload and longer lifetime increase the revenue that is generated by the spacecraft.
A key component of an ion thruster is its ion-optics system which is configured of multiple, closely-spaced electrodes that extract an ion beam from a plasma source. In a typical three-electrode system, the electrodes are referred to as a screen grid, an accelerator grid and a decelerator grid. An array of aperture sets are formed by these grids with each aperture set including one aperture of each of the three grids.
Voltages on the grids cause each of the aperture sets to extract ions from the plasma source and eject them as an ion beamlet. The ion beamlets combine to form an ion beam which is accelerated away from the ion-optics system. The ion beam's momentum generates an opposite force upon the ion thruster and attached structures (e.g., a spacecraft).
To enhance the ion beam's thrust and the thruster's lifetime, the alignment of each aperture set must be precisely established and maintained. An ion thruster's performance is a function of its current extraction which, in turn, is a function of ion beamlet alignment and, hence, of aperture set alignment. Thruster lifetime is degraded when aperture set misalignment causes ion beamlets to intersect an electrode and damage it by sputtering material from the electrode. This sputtered material can also shorten spacecraft lifetime if it deposits on sensitive surfaces (e.g., solar cells).
Alignment maintainence over temperature is typically enhanced by forming each of the electrodes with a spherical shape to reduce temperature-induced inter-grid movement. For example, each electrode in an exemplary 13 centimeter diameter ion thruster comprises a molybdenum sheet having a thickness in a range of 0.25-0.50 millimeters and a radius of curvature of .about.50 centimeters. The electrodes of this exemplary thruster form 3145 aperture sets.
Aperture set alignment is a function of the positional accuracy of each electrode's apertures. In the conventional electrode fabrication process illustrated in FIGS. 1A-1F, this positional accuracy has been found to be limited to .about.125 micrometers. This process begins with a circular molybdenum sheet 20. Because the sheet is quite thin, a portion of the sheet within an ellipse 22 is enlarged in each of FIGS. 1A-1E and shown immediately above the sheet 20 to illustrate process details.
In FIG. 1A, the sheet 20 is first coated with a negative light-sensitive photoresist 24 to form a coated electrode 26. In FIG. 1B, a mask 28 (e.g., a patterned photographic plate) is placed over the photoresist 24 to form an electrode assembly 30. The mask is designed to cover aperture portions 32 and to not cover web portions 34 of the sheet 20. Next, the electrode assembly 30 is uniformly radiated with radiation 36 so that the photoresist portions 38 which are not covered by the mask 28 are exposed to the radiation. In a negative photoresist, the unexposed portions can then be dissolved away to form the prepared sheet 40 of FIG. 1C in which only the photoresist portions 38 remain over the sheet web portions 34.
The prepared sheet 40 is next deformed from its shape in FIG. 1C to realize a prepared spherical electrode 44 that is shown in FIG. 1D (the force arrows 42 of FIG. 1C exemplify the deforming process). Accordingly, the enlarged structure within the ellipse 22 of FIG. 1D now indicates a radius of curvature. A chemical etchant 46 is applied in FIG. 1D and the aperture portions 32 are etched away to form apertures 48 that are separated by web portions 34 as shown in the etched spherical electrode 50 of FIG. 1E. In a final step, the photoresist portions 38 are removed to realize the apertured spherical electrode 52 of FIG. 1F which has an array 54 of apertures 48 that are separated by webs 34.
It has been determined that the deformation of FIGS. 1C and 1D stretches the sheet 20 which alters the locations of the photoresist portions 38 of FIG. ID in an unpredictable and uncontrollable manner. Consequently, the positional accuracy of the apertures 48 of FIG. 1E is limited, as recited above, to .about.130 microns and electrodes fabricated with the conventional process of FIGS. 1A-1F degrade the aperture set alignment of ion-optics systems.